1. Field of the Invention
This invention relates to a continuous process of smelting metallic lead directly from lead-and sulfur-containing materials in an elongated horizontal reactor, wherein a molten bath consisting of a slag phase and a lead phase is maintained in the reactor the charge is fed into the reactor on one side thereof onto the molten bath in a melting zone so as to maintain an oxidation potential which causes metallic lead and slag to be formed, reducing agents are introduced into the slag phase on the other side of the reactor in a reducing zone, and low-lead slag and metallic lead are tapped from their respective phases.
2. Discussion of Prior Art
German Offenlegungsschrift No. 28 07 964 discloses such a continuous process of converting lead sulfide concentrates into a liquid lead phase and a slag phase under a gas atmosphere having SO.sub.2 --containing zones in an elongated horizontal reactor. In that known process, lead sulfide concentrates and fluxes are charged onto a molten bath. The lead phase and a slag phase having a low non-ferrous metal content are discharged at mutually opposite ends of the reactor, and the phases flow countercurrently to each other in substantially continuous layer-forming streams to the outlet ends. At least part of the oxygen is blown into the molten bath from below through a plurality of mutually independently controlled nozzles, which are distributed over the length of the oxidizing zone of the reactor. The solid charge is charged into the reactor in several stages through a plurality of mutually independently controlled feeders, which are distributed over a substantial length of the reactor.
The locations and rates at which oxygen and solids are fed are so selected that the gradient of the oxygen activity in the molten bath has at the lead discharge end a maximum for the production of lead and from said maximum decreases progressively to a minimum for the production of slag phase having a low non-ferrous metal content, which minimum is obtained at the end where said slag phase is discharged. Gaseous and/or liquid protective fluids are blown into the molten bath at controlled rates together with the oxygen and serve to protect the nozzles and the surrounding lining and to assist the control of the process temperature. The rates at which gases are blown into the molten bath are so controlled that the resulting turbulence is suffficient for a good mass transfer but will not substantially disturb the flow of the phases in layers and the gradient of the oxygen activity. The gas atmosphere in the reactor is conducted countercurrently to the direction of flow of the slag phase. The exhaust gas is withdrawn from the reactor at the outlet end for the phase having a high non-ferrous metal content.
German No. 24 17 978 discloses such a process in which the gas atmosphere is conducted cocurrently to the slag phase.
U.S. Pat. No. 3,663,207 discloses a direct lead-smelting process in which the slag phase and lead phase are cocurrently conducted through the reactor, the slag is tapped at one end of the reactor and the lead is tapped from an intermediate zone of the reactor.
"Engineering Mining Journal," April, 1978, pages 88 to 91, describes on page 118 a direct lead-smelting process in which the fine-grained concentrate is ignited in a vertical shaft in the presence of oxygen and in fluidized state is roasted, smelted and partly reduced to metallic lead. From a hearth furnace under the shaft, the molten material flows under a partition into a hearth chamber, which is subjected to electric resistance heating and in which non-ferrous metal oxides are reduced to liquid lead. Slag and lead are tapped from that hearth chamber.
In these known direct lead-smelting processes, the crude lead which is tapped contains the entire bismuth. Bismuth is an impurity, which must be removed from the final product (refined lead) at high cost, and is also a by-product, which has a commercial value. A large part of the refined lead which is produced can be sold with Bi contents of 100 ppm and more. In certain grades, however, 70 ppm Bi or even less must not be exceeded. Where high-Bi lead concentrates are used, the refining costs involved in the removal of bismuth are usually more than offset by the commercial value of that metal but with raw materials having a low Bi content the refining costs exceed the proceeds. For this reason, numberous lead smelters separate their high-Bi and low-Bi raw materials and process them in separate batch operations. That practice results in numerous difficulties in the smelting and refining operations and also in a loss of interest on capital, particularly when concentrates high in noble metals must be stored for a long time.
It is also known to extract metallic lead from lead sulfide ores by the roast-re-acting process stages. In the first stage, part of the lead content is extracted as primary metallic lead and a high Pb slag is formed. The high-Pb slag is tapped and charged to the second stage, in which the secondary metallic lead is extracted and a low-Pb slag is formed under reducing conditions. The primary lead recovered in the first stage includes a major part of the bismuth contained in the charge. As a result, the secondary lead is low in Bi (German No. 589,738; German No. 590,505; German Offenlegungsschrift No. 27 39 963). These processes are not suitable for the continuous processes of the kind described first hereinbefore, in which lead is directly smelted in a single stage.
It is an object of the invention to provide a continuous process of directly smelting lead in a single stage in which the bismuth content of the charge is collected in the smallest possible quantity of crude lead.